A boat propulsion system using an outboard motor generally includes a propeller driven by a powerhead to propel the boat through the water. A type of outboard motor that discharges exhaust through the propeller is referred to as an exhaust-through-the-hub motor.
In an outboard motor that includes a pump jet system, an axial-flow pump jet system is driven by a powerhead of an engine. An impeller or rotor is mounted (e.g., spline fitted) directly on the propeller output shaft in place of the propeller and is encased within a housing. Such a system has the advantages of reducing hazards to swimmers in the vicinity of the motor, protecting the rotating elements from interference with and damage by foreign objects in the water, and improving the efficiency and performance of the propulsion system. Another benefit inherent with the pump jet is a directed jet of water that results in greater steering response.